This invention relates to the polymerization and copolymerization of a mono-1-olefin monomer, such as ethylene, with a higher alpha-olefin comonomer.
Supported chromium catalysts long have been a dominant factor in the production of high density olefin polymers, such as polyethylene. As originally commercialized, these catalyst systems were used in solution polymerization processes. However, it became evident early that a slurry process was a more economical route to many commercial grades of olefin polymers, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent.
It is well known that mono-1-olefins, such as ethylene, can be polymerized with catalyst systems employing vanadium, chromium or other metals on a support, such as alumina, silica, aluminum phosphate, titania, zirconium, magnesium and other refractory metal supports. Initially, such catalyst systems primarily were used to form homopolymers of ethylene. Soon copolymers were developed wherein comonomers such as propylene, 1-butene, 1-hexene or other higher mono-1-olefins were copolymerized with ethylene to provide resins tailored to specific end uses.
Often, high density and/or high molecular weight copolymers can be used for blow molding applications because the blow molding process enables rapid processing into a desired molded product. Theoretically, any type of resin can be made to flow more easily by merely lowering the molecular weight, (i.e., by raising the melt index.) However, this is rarely practical because of other penalties that occur because of a higher melt index (MI). A higher melt index can result in a decrease in melt strength, which can cause a parison to tear or sag during extrusion because the parison is unable to resist its own weight. As used in this disclosure, a parison is an extruded cylinder of molten polymer before it is blown by air pressure to fill a mold. Additionally, a higher MI can cause bottle properties such as environmental stress crack resistance (ESCR) and impact strength to decrease. One of the most prevalent problems associated with raising the MI is an increase of the amount of swell exhibited by the resin as it exits the die.
Two kinds of swell are critical during blow molding. These are "weight swell" and "diameter swell"; the later also is referred to herein as "die swell". As polymer, or resin, is extruded under pressure through a die opening and into a mold, a polymer has a tendency to swell as it exits the die. This is known as weight swell and is determinative of the thickness of bottle wall, as well as the overall weight of the resultant blow molded product. For example, a resin which is extruded through a 0.02 inch die gap might yield a bottle wall thickness of 0.06 inches, in which case the weight swell is said to be 300%. A resin that swells too much can produce a bottle with too thick of a wall. To compensate, the die opening or gap can be narrowed by manual adjustment. However, any decrease in die gap can increase the resistance to the flow of the resin through the die. Narrower die gaps can result in higher shear rates during extrusion which also can increase in melt fracture leading to a rough bottle surface. Thus, a resin which can be described as easily processable must exhibit low weight swell, which allows a wide die gap.
Diameter, or die, swell refers to how much the parison flares out as it is extruded from the die. For example, a resin extruded through a circular die of one (1) inch diameter can yield a parison tube of 1.5 inches in diameter; the die swell is said to be 50%. Die swell is significant because molds usually are designed for a certain amount of flare; too much die swell can interfere with molding of a bottle handle. A high degree of weight swell often causes high die swell because of the narrow gap that accompanies it. Unfortunately, increasing the melt index of a resin usually increases both weight swell and die swell of the polymer. Thus, as used herein, a resin which is considered easily processable also should exhibit low die swell.